The present invention relates to a foreign substance inspecting method and apparatus suitably used in lithography (e.g., in Proximity X-ray Lithography: to be referred to as PXL hereinafter) with which a mask pattern is exposed and transferred with a one-to-one exposure onto a wafer arranged close to the mask by using, as a light source, X-rays with a wavelength of 7 xc3x85 to 10 xc3x85 and output from an electron storage ring (to be referred to as an SR hereinafter) serving as a synchrotron radiator, and an exposure apparatus using this inspecting method.
More particularly, the present invention relates to a foreign substance inspecting method and apparatus which cope with the problem of a foreign substance specific to PXL exposure with which exposure is performed by separating a mask and wafer from each other by a very small distance of several tens of xcexcm or the like, and an exposure apparatus using this inspecting method.
PXL is a micropattern exposure technique with which a mask is set to oppose a wafer at a gap of 10 xcexcm to 30 xcexcm and a pattern on the mask is transferred to the wafer by Fresnel diffraction.
As a PXL type exposure apparatus, currently, one with a maximum exposure range of 52 mm square is expected to be manufactured. As this exposure apparatus performs one-to-one exposure with the maximum exposure range of 52 mm square, if exposure is to be performed on a wafer with a size of 4 inches or more, the entire surface of the wafer cannot be exposed by one exposure operation. For this reason, exposure is performed while sequentially moving the wafer so that the entire surface of the wafer is exposed, as with a repetitive stepper for an optical exposure apparatus. Hence, a PXL exposure apparatus is sometimes called a one-to-one exposure X-ray stepper.
The characteristic feature of the PXL exposure apparatus resides in its high resolution. With a high resolution of 100 nm or less and alignment of 20 nm or less having already been reported, the PXL exposure apparatus may become a leading exposure method for a 1-GDRAM or RAMs with capacities larger than that. One of the special items of the PXL is an X-ray mask. A conventional manufacturing process for an X-ray mask will be described with reference to FIGS. 9 to 19. Note that in FIGS. 9 to 19, for descriptive convenience, the thicknesses of the films are illustrated with a proportion different from that of actual films.
In fabrication of the X-ray mask, first, as shown in FIG. 9, a Si wafer 30 is prepared as a substrate and, as shown in FIG. 10, a SiC film 31 called a membrane and with a thickness of 2 xcexcm to 3 xcexcm is formed on it. When forming the SiC film 31 on the Si wafer 30, SiC films are formed on the upper and lower surfaces and side surfaces of the wafer. As the SiC films on the lower surface and side surfaces of the wafer are not related to the function of the mask, they are omitted in FIG. 10 and so on.
In FIG. 11, the SiC surface is polished to form a planar SiC film 32, and an ITO film or SiO2 film 33 is formed, as shown in FIG. 12. In the step of FIG. 13, an X-ray absorber 34, e.g., W, Ta, or Ta4B, having a relatively high X-ray absorption performance is formed to a thickness of 0.3 xcexcm to 0.5 xcexcm. In FIG. 13, preparation of the substrate is completed.
In the step of FIG. 14, a resist is applied to the substrate, and a desired pattern is drawn on the substrate with an electron beam drawing unit. Then, the substrate is subjected to development, etching, and resist removal, thereby forming a pattern. When the pattern is formed, the opposite side to the pattern portion is etched back so X-rays can be transmitted through a Si portion 35 within the exposure range. Finally, in FIG. 16, the substrate is mounted on a frame 36, thus completing an X-ray mask.
In order to decrease drawing errors, the frame 36 in the state shown in FIG. 13 may be mounted as shown in FIG. 17, and Si in the portion 31 within the exposure range may be etched back, so X-rays can be transmitted through this portion. After that, a resist is applied to the substrate, and a desired pattern is drawn on the substrate with an electron beam drawing unit. Then, the substrate is subjected to development, etching, and resist removal, thereby forming a pattern. The X-ray mask shown in FIG. 19 is thus completed. It is known that this method has a high precision since the substrate is finally mounted on the frame.
In fact, however, if the substrate is subjected to the drawing process after it is mounted on the frame, the mount may be peeled due to heat. Therefore, conventionally, the X-ray mask is usually formed with the steps of FIGS. 9 to 16. The frame 36 is sometimes called a support ring, and is made of, e.g., Pyrex or SiC. To mount the membrane, anodic bonding or an adhesive is used. Another method is also proposed in which, as shown in FIG. 17, the frame is also made of Si to form an integral frame 37, and an Si wafer substrate and the frame are formed integrally.
In the PXL exposure apparatus, since a membrane with a thickness of 2 xcexcm to 3 xcexcm is used as a mask, it has a specific problem in that a foreign substance with a size equal to or larger than the exposure gap is sandwiched between the wafer and mask (particularly, a SiC membrane) to come into contact with them, thereby fracturing the SiC portion of the mask.
Assuming that a typical exposure gap is 10 xcexcm, the existence of a foreign substance with a size equal to or larger than 10 xcexcm between the wafer and mask may seem nonsensical in semiconductor manufacture where the idea of high yield prevails. This size, however, cannot be ignored as the size of a foreign substance occurring on the periphery of a wafer.
Most exposure apparatuses used in present semiconductor manufacture are optical exposure apparatuses, and a foreign substance attaching to the periphery of a wafer does not pose an issue. In an optical exposure apparatus, since the distance between the wafer and the projection optical system of the exposure apparatus is at least 1 cm, the problem of the contact of a foreign substance does not arise, and the situation is completely different from that of the PXL exposure apparatus. Furthermore, since the periphery of the wafer is not used for formation of ICs, an inspection for a foreign substance on the periphery of the wafer is not conventionally performed.
The present inventors performed observation of the peripheries of various types of wafers. It has become apparent that many large-sized foreign substances are present on the periphery of a wafer even in the semiconductor manufacture where the idea of high yield prevails. However, due to the reason described above, a foreign substance on the periphery of a wafer does not pose a serious issue in the conventional optical exposure apparatus. Even in the optical exposure apparatus, a foreign substance can move from the periphery of a wafer and shift onto the wafer pattern, thus causing a problem. As a countermeasure for this, inspection is performed by using a wafer foreign substance inspecting apparatus for detecting a foreign substance on a patterned wafer, so that a decrease in yield is prevented.
Still, in the PXL exposure apparatus, unlike in the optical exposure apparatus, a foreign substance on the periphery of a wafer can cause fracture of a mask. This is a serious problem.
FIGS. 20 to 22 are views for explaining a phenomenon that occurs when a mask 1 is exposed at a predetermined gap with a foreign substance 13 attaching to the periphery of a wafer 2. In FIG. 21, after the wafer moves, when a portion near the periphery of the wafer is exposed, a force acts on the foreign substance 13 attaching to the wafer 2. In the state of FIG. 21, since the foreign substance 13 is in contact with that portion of the mask 1 where the Si portion is not etched back, it does not fracture the mask 1.
After the shot shown in FIG. 21 is exposed, when the wafer 2 is moved so as to expose another portion, a force acts on the foreign substance 13. As shown in FIG. 22, upon movement of the wafer 2, the foreign substance 13 is separated from the wafer 2 and is moved to attach to the SiC portion of the etched-back transfer pattern portion 35 of the mask 1. If exposure and movement of the wafer are repeated in this manner, a force acts on the foreign substance 13 again, and sometimes SiC, with a thickness of 2 xcexcm to 3 xcexcm, may be fractured.
A case wherein a foreign substance attaches to the periphery of a wafer has been described. The same phenomenon occurs when a foreign substance attaches to the periphery of a mask. FIGS. 23 and 24 show this case. First, in the initial shot of wafer exposure, the foreign substance 13 attaching to the mask 1 gets sandwiched between the mask 1 and wafer 2, and a force acts on the mask 1. At this time point, the mask 1 is not fractured in the same manner as in the case shown in FIG. 21 wherein a foreign substance attaches to the periphery of a wafer. Subsequently, when the wafer 2 moves, the foreign substance 13 is moved to a SiC portion of the mask 1 where the Si portion is not etched back, and the mask 1 is fractured undesirably, as shown in FIG. 24.
As described above, in the PXL exposure apparatus, a foreign substance attaching to the periphery of a wafer or mask, particularly, a large foreign substance with a size equal to or larger than the exposure gap which does not pose an issue in the optical exposure method, causes mask fracture, and is accordingly a serious problem.
Even if the mask is not fractured, when the foreign substance which has moved to the wafer and attached to it is not detected by another inspection, the original function of the semiconductor cannot be achieved, and a decrease in yield results. Conventionally, however, the possibility of movement of a foreign substance in an optical exposure apparatus is smaller than that in a PXL exposure apparatus, and a foreign substance on the periphery of a wafer is out of interest.
The present invention examined foreign substances on the entire surfaces of a wafer and an X-ray mask, including their peripheries, by using an existing wafer foreign substance inspecting apparatus for inspecting a foreign substance on a patterned wafer. As the existing wafer foreign substance inspecting apparatus, for example, one which illuminates a wafer with polarized light by oblique incidence exists. The detection principle of this method utilizes a feature that a circuit pattern reflects light with the polarization characteristics being maintained, while a foreign substance reflects light in a non-polarized state. This detection system has been manufactured as a product. Such a product has a high throughput with an actual detection time of 1 minute or less with an 8-inch wafer, and accordingly a high reliability, thus contributing to a high yield in semiconductor manufacture.
According to the studies of the present inventors, when the above wafer foreign substance inspecting apparatus is used, a foreign substance on a mask or wafer can be discriminated from a notch in Si in the periphery and can accordingly be detected. Meanwhile, in the vicinity of the periphery of the mask, even when no foreign substance is present, a false signal corresponding to the size of an ordinary foreign substance or larger than that is sometimes generated, and this false signal may be erroneously detected as a large-sized foreign substance.
It is apparent that a false signal is generated because a mask is not conventionally manufactured by paying attention to the structure of is periphery. Various types of foreign substances caused by mask handling exist on the mask periphery, e.g., a foreign substance attaching to the periphery when the wafer is held by a CVD (Chemical Vapor Deposition) unit, variations in thickness of SiC or an absorber due to the influence of the surrounding atmosphere, or film removal occurring as the film is scratched by tweezers when the mask is to be mounted on the frame.
When the X-ray mask is measured with a conventional foreign substance inspecting apparatus for a patterned wafer, typically, a foreign substance like those described above may be determined as being a large-sized foreign substance with a size of 10 xcexcm or more. The existing wafer foreign substance inspecting apparatus uses a detection principle that a foreign substance scatters light by isotropic scattering, and determines the size of a foreign substance based on a correlation table with respect to signal outputs stored in advance. If the film is peeled or film thickness varies, light is refracted and scattered through a complicated process, so the polarized light rotates. Thus, a large signal output is detected when compared to a case wherein a foreign substance which has the same size as that described above and which causes isotropic scattering is present. For example, when the film is peeled, this may be erroneously determined as being a large-sized foreign substance with a size of 10 xcexcm or more, which is larger than the actual size.
It is also apparent that, on the wafer periphery, a foreign substance is crushed during transfer or while the wafer is being mounted on a carrier, and a large-sized, but not tall, foreign substance is detected. A peeled film, a nonuniform-thick film, or a flat foreign substance on the wafer, however, is not a foreign substance with a height of 10 xcexcm or more, which can fracture the mask and must accordingly be solved by the present invention.
It is an object of the present invention to provide a foreign substance inspecting method and apparatus which can realize exposure in connection with a foreign substance detecting function for discriminating a foreign substance that really poses an issue in a PXL exposure apparatus from a harmless foreign substance, and an exposure apparatus using this inspecting method.
The foreign substance inspecting method and apparatus according to the present invention, and the exposure apparatus using this inspecting method, are devised in order to solve the problem of a foreign substance specific to the PXL described above.
More specifically, the present invention is characterized in that, during exposure which is performed by separating a mask and wafer from each other by a predetermined distance, wafer foreign substance inspection for detecting the surface of the wafer and a foreign substance simultaneously, thereby detecting whether or not a foreign substance with a height equal to a preset value or more is present, is performed in a PXL exposure apparatus, a coater/developer, or the like.
For example, the wafer foreign substance inspecting apparatus can employ a method with which the periphery of a wafer is irradiated with light in a direction almost tangent to the outer circumference of the wafer, then the irradiation light is reflected and returned to an optical path it has passed along in order to irradiate the periphery of the wafer again, and the irradiating light is received.
Conventionally, an existing foreign substance inspecting apparatus for a patterned wafer determines the size of a foreign substance based on a detection signal and a correlation table showing the relationship between a prestored signal output and a foreign substance. In other words, the size of a foreign substance is merely determined from the magnitude of a signal output, thereby measuring the size of the foreign substance. To detect a foreign substance that might fracture a mask, the height of the foreign matter, which is the most significant parameter, is not detected. Accordingly, to simply apply the conventional foreign substance inspecting apparatus, which detects the size of a foreign substance from a signal output, to a PXL exposure apparatus is not sufficient. This problem can be solved only when the height of the foreign substance is detected directly.
In a wafer foreign substance inspecting method and apparatus according to the present invention, and an exposure apparatus using this inspecting method, a foreign substance inspecting apparatus, using a detection method capable of detecting the height of a foreign substance on the periphery of a wafer in question with a high throughput, is prepared. This foreign substance inspecting apparatus is applied to a PXL exposure apparatus, so the problem of foreign substance specific to the PXL is solved.
More specifically, with a foreign substance inspecting method according to the present invention, a height of a foreign substance attaching to a periphery of a wafer is detected, thereby detecting the presence/absence of a foreign substance with not less than a predetermined height.
The foreign substance inspecting method according to the present invention is applied to an exposure apparatus that exposes a mask and wafer separated from each other at a predetermined distance, thereby detecting a height of a foreign substance attaching to the wafer.
The foreign substance inspecting method according to the present invention is applied to an exposure apparatus that exposes a mask and wafer separated from each other at a predetermined distance, thereby detecting a size and height of a foreign substance attaching to the wafer.
A foreign substance inspecting apparatus inspecting a foreign substance apparatus attaching to a wafer according to the present invention comprises an irradiation unit for irradiating a periphery of the wafer with light in a direction substantially tangent to an outer circumference of the wafer while rotating the wafer, and a detection unit for detecting the presence/absence of a foreign substance with not less than a predetermined height on the basis of the irradiating light.
An exposure apparatus according to the present invention exposes a mask and wafer separated from each other at a predetermined distance, and comprises a foreign substance inspecting apparatus having an irradiation unit for irradiating a periphery of a wafer with light in a direction substantially tangent to an outer circumference of the wafer while rotating the wafer, and a detection unit for detecting the presence/absence of a foreign substance with not less than a predetermined height on the basis of the irradiating light.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such an example, however, is not exhaustive of the various embodiments of the invention, and, therefore, reference is made to the claims which follow the description for determining the scope of the invention.